U.S. Pat. Nos. 5,476,572; 5,622,598; 5,635,025; 5,766,418; and 5,968,314 disclose methods and devices for feeding a slurry of comminuted cellulosic fibrous material to a treatment vessel that have revolutionized the art of treating comminuted cellulosic fibrous material to produce cellulose pulp. The disclosed inventions, sold under the trademark LO-LEVEL(copyright) by Ahlstrom Machinery Inc., of Glens Falls, N.Y. employ one or more slurry-type pumps for treating and transferring comminuted cellulosic material to one or more treatment vessels. Not since the initial development of the continuous cooking process in the 1940s and 1950s have such dramatic improvements been made to equipment used to transfer material to a treatment vessel, for example, a continuous or batch digester. This is confirmed by the broad acceptance of this technology by the Pulping Industry.
The present invention introduces improvements to the systems and methods described in the above patents which further simplify and enhance the effectiveness of the methods and devices disclosed therein.
The prior art systems for introducing a slurry of comminuted cellulosic fibrous material, for example, as exemplified by the system disclosed in U.S. Pat. No. 5,476,572, use a two-stage pressurization and transfer of slurry. In the first stage, the slurry is pressurized to a first pressure and transferred to a high-pressure transfer device, such as, a High Pressure feeder designed and marketed by Ahlstrom Machinery. The first stage pressurization and transfer is typically performed using a specially-designed slurry pump which handles slurries of material and liquid. In the second stage the High Pressure Feeder pressurizes the slurry to a second pressure, higher than the first pressure, by exposing the material to a high pressure liquid stream, and transports the slurry to a treatment vessel, for example, a continuous or batch cellulose pulp digester. However, according to this prior art, the amount of cellulose material, such as, wood chips, that can be transferred to the High Pressure Feeder by the slurry pump, per unit volume of liquid, is limited by the capacity of the pump to transfer solid material.
Typically, the relative amount of liquid present in slurry is indicated by a xe2x80x9cliquid-to-solidsxe2x80x9d ratio, or, in the case of transferring slurries of wood chips, a xe2x80x9cliquid-to-chipxe2x80x9d ratio, or, more specifically, a xe2x80x9cliquor-to-woodxe2x80x9d (L/W) ratio. The liquid-to-wood ratio is a dimensionless ratio of the volume of the liquid present in the slurry to the volume of the wood present in the slurry. Conventional High Pressure Feeders can accept slurries having L/W of below 3.0:1, typically even below 2.5:1. The lower limit of the L/W ratio of a slurry being introduced to a High Pressure Feeder is about 2.0:1. Note that a reduction in L/W ratio from 3.0:1 to 2.0:1 corresponds to a 25% reduction in the volume of liquid that must be accepted by the High Pressure Feeder, or a corresponding 25% increase in the volume of chips that can be processed by the High Pressure Feeder.
Desirably the volume of liquid that is transferred to the High-pressure Feeder is reduced so that more wood chips can be introduced and processed in the digester system being fed per revolution of the High Pressure Feeder. This has the further advantage of allowing for the reduction in size of the High-pressure Feeder for a given project, or allowing for an increase in the capacity of a production-limited facility.
After introducing the slurry of chips to a high-pressure transfer device, for example, a High-Pressure Feeder sold by Ahlstrom Machinery, the slurry is displaced from the feeder by a flow of high-pressure liquid, typically at a pressure between about 5 and 15 bar gage, provided by a high-pressure pump. Typically this flushing of the slurry from the feeder by the liquid results in the slurry being propelled to a treatment vessel having a L/W ratio of between about 4.0:1 to 10.0:1, and is typically greater than 5:1, often greater than 7:1, sometimes greater than 9:1. For example, for a L/W ratio of 9:1, the volume of liquid present in the conduit transferring the slurry from the feeder to the treatment vessel, for example, to a pulping digester, the volume of liquid is 9 times the volume of the cellulose material, such as, wood chips. Typically, this volume of liquid is required in order to flush the chips from the pockets of the feeder. This relatively large volume of liquid requires a relatively large conduit in which to pass the slurry from the feeder to the digester and sufficient energy to propel the relatively large volume of liquid up to the top of the pressurized digester.
The L/W ratio of the slurry exiting the High Presser Feeder is also a function of the equipment which feeds the slurry to the feeder. In conventional, xe2x80x9csuck throughxe2x80x9d systems typically having a pressurized chip chute the L/W ratio of the slurry introduced to the High Pressure Feeder is about 2.0-2.5:1. In xe2x80x9cpump throughxe2x80x9d systems, such as Lo-Level Feed Systems sold by Ahlstrom Machinery, the L/W ratio of the slurry introduced to the High Pressure Feeder is about 3.0-3.5:1.
Desirably the liquid volume in the slurry transferred from the feeder to the treatment vessel is minimized by removing at least some of the liquid from the slurry after the slurry has been discharged from the feeder and before the slurry is introduced to the treatment vessel. One advantage of this embodiment of the invention is that, with reduced liquid volume, the diameter of the transfer conduit to the treatment vessel can be reduced. Reducing the size of this conduit has the further advantage of reducing the sizes, and hence the cost, of the associated valves and instruments that are located in this conduit.
The above embodiment is particularly effective in limiting the amount of heat returned to the feed system from the treatment vessel, for example, via what is known as the xe2x80x9cTop Circulationxe2x80x9d or xe2x80x9cTCxe2x80x9d line. As recognized in the art, exposing the feed system, for example, the High-pressure Feeder, to liquids having temperatures at or above 100xc2x0 C. can cause flash-evaporation of this liquid (known as xe2x80x9cflashingxe2x80x9d) when the liquids are exposed to the atmospheric pressures present in the vicinity of the high-pressure feeder. However, when excess liquid is removed from the slurry when introducing the slurry to the treatment vessel, for example, by using of a Top Separator, heat present in the treatment vessel can migrate, for example, by convection, to the vicinity of the Top Separator and be drawn out of the vessel with the removal of liquid from the Top Separator. This heat can raise the temperature of the liquid returned to the feed system via the TC line. This increased TC line temperature can cause flashing and vibration in the feed system and interfere with the normal operation of the feed system.
One way of reducing the potential of returning undesirable heat to the feed system is by limiting the flow of liquid removed from the slurry as the slurry is introduced to the treatment vessel. That is, a liquor removal device is located in the conduit which feeds the slurry to the treatment vessel, preferably, near to or adjacent the inlet of the treatment vessel. At least some liquid is removed from the slurry using this device and returned to the feed system such that less liquid needs to be removed from the slurry as the slurry is introduced to the vessel. This reduced removal of liquid from the vessel reduces the potential for heat in the vessel to be withdrawn with the removed liquor and returned to the feed system.
One liquid separating device that is novel according to the invention, and that is particularly useful in the system and practice of the method of the present invention, is a cylindrical device having a cylindrical screen through which the slurry passes and from which liquid is removed, for example, an In-line Drainer, as sold by Ahlstrom Machinery Inc. of Glens Falls, N.Y. An In-line Drainer is typically used to isolate a stream of liquid from a stream of liquid that typically contains at least some wood chips or fine wood particles, for example, what are known as xe2x80x9cfinesxe2x80x9d and xe2x80x9cpinsxe2x80x9d. However, an In-line Drainer can also be used in the practice of the present invention where a liquid is preferably removed from a slurry containing a larger amount of cellulose material, in particular wood chips.
In the conventional use of an In-line drainer, the drainer is positioned in a feed system of a continuous digester, for example, in the outlet of a Sand Separator [as shown by item 37 in FIG. 2 herein]. The liquid passed to the drainer from the Sand Separator can typically contain at least some wood particles or other material. The In-Line Drainer is typically used to remove excess liquid from the low pressure liquor circulation associated with the feed system, that is, the Chip Chute Circulation, to control the volume of liquid, for example, in the Chip Chute or Chip Tube. Conventional drainers include cylindrical screen baskets fashioned from steel bars oriented parallel to the direction of flow so that the liquid passes through vertical slots or apertures while retaining wood particles within the circulation. However, due to the low concentration of chips, pins, and fines in the liquid passing through the drainer, the flow of liquid through the basket is such that the chips, pins, and fines are oriented in the direction of flow which is also parallel to the slots in the basket. As a result, without taking appropriate measures, the chips, pins, and fines can align with and undesirably pass through the vertical slots or become lodged in the vertical slots of the drainer.
In the drainer of the conventional art, for example, as shown in FIG. 5 herein, the potential for chips, pins, and fines to align with and pass through the vertical slots of the drainer basket is minimized by introducing a horizontal velocity component to the liquid flow as it is passes through the drainer. This is typically achieved by introducing a helical baffle, or so-called xe2x80x9cflightxe2x80x9d, to the inlet of the drainer in order to impart a helical flow to the liquid as it is introduced to the drainer and passes through the drainer basket. Due to this helical flow, any chips, pins, or fines that may be present are oriented in the direction of the helical flow and thus oriented obliquely to the elongation of the slots of the vertical bars. Thus, in the conventional art, the helical flight in the inlet reduces the tendency for chips, pins, and fines to pass through the drainer basket or to be lodged in the slots of the drainer basket and cause pluggage of the drainer.
Though this conventional In-line Drainer has proven to be very effective in most applications, the flight positioned in the inlet of the conventional drainer has, in some applications, been associated with an undesirable pressure drop across the drainer. That is, the helical baffle introduces an impediment to flow which causes a decrease in hydraulic pressure from the pressure of the liquid introduced to the drainer to the pressure of the liquid leaving the drainer. This pressure drop impedes the flow of liquid through the drainer and also reduces the pressure of the liquid downstream of the drainer, which can interfere with the proper operation of downstream equipment, for example, the Level Tank or Make-up Liquor Pump. This flow impediment can also reduce the velocity of the flow and thus increase the likelihood for chips, etc. to pass through or become lodged in the screen.
According to the present invention, the helical baffle present in the inlet of prior art In-line Drainers and the source of pressure drop associated with this baffle are eliminated, yet the Drainer still functions properly. To account for the loss of the baffle""s function, according to the present invention, the slots or apertures of the screen basket are aligned obliquely to the direction of elongation of the drainer, and thus obliquely to the direction of flow of the liquid through the drainer. The angle of the slots relative to the direction of elongation of the screen can range from between about 5 to 90 degrees. For example, in one embodiment the slots are oriented substantially perpendicular to the direction of elongation and direction of flow. In the preferred embodiment, the slots are oriented at an angle of about 10xc2x0 to 80xc2x0, preferably about 30xc2x0 to 60xc2x0, most preferably about 40xc2x0 to 50xc2x0.
One embodiment of the present invention consists or comprises a liquid separating device having a cylindrical housing elongated in a direction of elongation having an inlet at or adjacent a first end of the housing, an outlet at or adjacent a second end, opposite, the first end, and an inside surface; a cylindrical screen assembly centrally mounted in the housing having a plurality of elongated apertures having an angle of orientation and an outside surface; an annular cavity formed by the outside surface of the screen and the inside surface of the housing; and an outlet for separated liquid located in the housing and communicating with the annular cavity; wherein the angle of orientation of the screen assembly apertures is oblique to the direction of elongation of the housing. The angle of orientation is preferably at least 5xc2x0 to the direction of elongation of the housing or screen basket, but is typically between about 10xc2x0 to 80xc2x0, preferably about 30xc2x0 to 60xc2x0, most preferably 40xc2x0 to 50xc2x0 to the direction of the elongation of the housing or screen basket. For example, the orientation of the slots relative to the elongation of the housing is about 45xc2x0.
The drainer slots may be continuous slots or they may be interrupted by unperforated xe2x80x9clandxe2x80x9d areas. These land areas may be uniformly located throughout the screen basket so that a uniform pattern of slots and land areas is provided or the slots and land areas may be distributed non-uniformly. The orientation of the slots may also vary, for example, the angle of orientation of the slots at one elevation in the direction of elongation of the screen basket may be different from the orientation of the slots at second or an adjacent elevation. The orientation of slots at one elevation in the direction of elongation of the screen basket may also vary, for example, producing a xe2x80x9cherring bonexe2x80x9d-type pattern of slots. The screen slot configuration of this device may be similar or identical to the screen designs illustrated and described in U.S. Pat. No. 6,039,841 or in co-pending application Ser. No. 09/248,005 filed on Feb. 10,1999, now U.S. Pat. No. 6,165,323 the disclosures of which are incorporated by reference herein.
The slots may be fabricated from parallel-bar-type or parallel-wire-type construction or they may be machined from plate, for example, by water-jet cutting, laser cutting, EDM machining, drilling, milling, or any other conventional method of producing apertures in plate. The housing or screen basket material is typically metallic, for example, steel, steel-based alloy, stainless steel, aluminum, titanium or any other commercially available metal, but may also be manufactured from a high-performance plastic or composite material.
The drainer according to the present invention may be used in a conventional feed system, as shown by item 37 in FIG. 2 of this application, or for treating slurries according to the method and apparatuses of the present invention, for example, as shown as items 153 and 154 of FIG. 3 or item 254 of FIG. 4.
That is, the invention comprises: A drainer for draining liquid from a moving slurry, comprising: An elongated housing having an inlet and an outlet at or adjacent opposite ends thereof; and a dimension of elongation between the inlet and outlet. The inlet substantially open and hollow. A substantially annular screen positioned between the inlet and the outlet having a plurality of slots therein. And the screen slots having an oblique inclination angle xcex1 with respect to the dimension of elongation between about 5-90xc2x0 to minimize passage of solid material in the slurry through the slots, and minimizing clogging of the slots. The angle xcex1 may be substantially 90xc2x0, or between about 10-80xc2x0, e.g. between 30-60xc2x0, or between about 40-50xc2x0 (e.g. about 45xc2x0).
The drainer may further comprise a plurality of land areas between regions of the slots. Also, the drainer preferably further comprises a substantially annular volume between the screen and housing, and a liquid outlet from the volume and a steam purge connected to the volume, and wherein the screen has a diameter of between about 0.5-3 feet, preferably about 8 to 24 inches.
In one embodiment the slots have a width of between about 2-4 mm, and the slots are substantially evenly spaced by about 3-4 mm. In another embodiment the slots have a width of between about 5-7 mm, and the slots are substantially evenly spaced by about 4-8 mm.
According to another aspect of the invention there is provided a pulp producing system comprising: A substantially upright digester. A feed system for feeding a slurry of comminuted cellulosic material to the digester. The feed system including a drainer which receives some liquid from a flow of slurry. And the drainer comprising: an elongated housing having an inlet and an outlet at or adjacent opposite ends thereof; a substantially annular screen positioned between the inlet and the outlet having a plurality of slots; and the screen slots having an oblique inclination angle xcex1 with respect to the dimension of elongation of between about 5-90xc2x0 to minimize passage of solid material in the slurry through the slots, and minimize clogging of the slots. In one embodiment the drainer is positioned in a location in the feed system wherein the slurry has a L/W ratio of greater than about 20:1, preferably greater than about 50:1 but includes at least some pins, fines, or chips, and a pressure of from about 0-5 bar gauge; and wherein the slots are evenly spaced by about 3-4 mm and have a width of between about 2-4 mm. In another embodiment the drainer is positioned in a location in the feed system wherein the slurry has a L/W ratio of less than 15:1, preferably less than about 10:1 and a pressure of about 0-30 bar gauge; and wherein the slots have a width of between about 5-7 mm, and the slots are substantially evenly spaced by about 4-8 mm.
The feed system may include a high pressure transfer device (e.g. a high pressure feeder) or the feed system may include one or more slurry pumps as disclosed in U.S. Pat. No. 5,753,075 or co-pending applications Ser. No. 09/063,429 filed Apr. 21, 1998 now U.S. Pat. No. 6,106,668 and Ser. No. 09/568,889 filed May 11, 2000 now U.S. Pat. No. 6,325,890 the disclosures of which are incorporated by reference herein.
A method is provided for feeding a slurry of comminuted cellulosic fibrous material to a treatment vessel comprising or consisting of: a) slurrying the material with a slurrying liquid to produce a slurry of material and liquid having a first liquid-to-material volume ratio; b) pressurizing the slurry to a first pressure and transferring the slurry to a high-pressure transfer device; c) introducing the slurry to the high-pressure transfer device; d) in the high-pressure transfer device, pressurizing the slurry to a second pressure, higher than the first pressure; e) transferring the slurry from the high-pressure transfer device to the treatment vessel; f) introducing the pressurized slurry to the treatment vessel; and g) removing at least some of the liquid from the slurry between a) and c), using the drainer described above, so that the slurry introduced to the high-pressure transfer device in c) has a second liquid-to-material ratio lower than the first ratio. In a preferred embodiment, at least some of the liquid removed during step g) is used as at least some of the slurrying liquid of step a). Preferably g) is performed immediately prior to c), but f) may be performed at any time after a).
The method also may further include h) treating the material in the treatment vessel to produce cellulose pulp, for example, by a continuous or non-continuous (that is batch) chemical pulping process. For example, those processes disclosed in U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; 5,662,775; 5,824,188; 5,849,150; and 5,849,151 and marketed by Ahlstrom Machinery under the trademark LO-SOLIDS(copyright).
The first liquid-to-material volume ratio is used in slurrying the material in a) is typically greater than 2.75:1, preferably about 2.75 to 3.25 to 1.0. This ratio is typically required in order for the slurry pump, for example, a Hidrostal(copyright) screw-type-impeller slurry pump manufactured by Wemco of Salt Lake City, Utah, or a pump provided by Lawrence Pumps Inc. of Lawrence, Mass., to operate properly. Though for other types of slurry pumps this L/W ratio may even be lower, for example, 2.50:1 or less. The second liquid-to-material ratio (that is, the ratio for the slurry introduced to the high-pressure feeder) is preferably about 2.50:1 or less, preferably about 1.75 to 2.25:1, or even less than about 1.75:1. In a preferred embodiment of this invention the second L/W ratio is at least 0.25 less than said first liquid-to-material ratio, most preferably at least 0.50 less than said first liquid-to-material ratio.
The first pressure to which the slurry is pressurized typically is in the range of 1 to 7 bar gage; the second pressure is typically in the range of 5 to 15 bar gage.
The present invention also includes a system for feeding comminuted cellulosic fibrous material to a treatment vessel, comprising or consisting of: a first vessel containing a slurry of comminuted cellulosic fibrous material having a first liquid-to-material volume ratio; a high-pressure transfer device having a low-pressure inlet, a low-pressure outlet, a high-pressure inlet, and a high-pressure outlet connected to the treatment vessel; means for pressuring and transferring the slurry from the first vessel to the low-pressure inlet of the high-pressure transfer device; a means for removing at least some of the liquid from the slurry located between the pressurizing means and the low-pressure inlet to provide a slurry having a liquid-to-material ratio less than the first ratio, the removal means comprising the drainer described above; and a means for transferring the slurry from the high-pressure outlet to the treatment vessel.
The first vessel is preferably a Chip Chute or Chip Tube provided by Ahlstrom Machinery. The high-pressure transfer device is preferably a High-pressure Feeder as sold by Ahlstrom Machinery. The means for pressurizing and transferring the slurry to the high-pressure transfer device may be a chip pump for pumping the slurry into the high-pressure transfer device or a pump (for example, a pump known as a Chip Chute Circulation Pump) for drawing the slurry into the high-pressure transfer device, or any other suitable conventional pressurizing device. A means for transferring the slurry from the high-pressure outlet of the high-pressure transfer device preferably comprises a high-pressure pump that provides pressurized liquid to the high-pressure inlet of the high-pressure transfer device. The preferred liquid-to-material ratios and pressures are preferably as described above.
Another aspect of the invention comprises a method of feeding a slurry of comminuted cellulosic fibrous material to a treatment vessel comprising or consisting of: a) slurrying the material with a slurrying liquid to produce a slurry of material and liquid having a first liquid-to-material volume ratio; b) pressurizing the slurry to a first pressure and transferring the slurry to a high-pressure transfer device; c) introducing the slurry to the high-pressure transfer device; d) in the high-pressure transfer device, pressurizing the slurry to a second pressure, higher than the first pressure using a pressurized liquid and to produce a slurry of liquid having a second liquid-to-material volume ratio, higher than the first ratio; e) discharging the slurry having the second volume ratio from the high-pressure transfer device; f) transferring the slurry to the treatment vessel; g) introducing the pressurized slurry to the treatment vessel; and h) removing at least some of the liquid from the slurry between e) and g) so that the slurry introduced to the treatment vessel in g) has a third liquid-to-material ratio lower than the second ratio, the liquid removal practiced using a drainer as described above.
In a preferred embodiment, at least some of the liquid removed during h) is used as the pressurized slurrying liquid for d). In another preferred embodiment at least some of the liquid removed during h) is used as the slurrying liquid in a). Also h) is preferably performed immediately after e) but h) may be performed at any time after e) but before g). The present invention also may further include i) treating the material in the treatment vessel to produce cellulose pulp, for example, by a continuous or non-continuous, that is batch, chemical pulping process. For example, those processes disclosed in U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; 5,662,775; 5,824,188; 5,849,150; and 5,849,151 and marketed by Ahlstrom Machinery under the trademark LO-SOLIDS(copyright). Also the method is preferably practiced to, between a) and c), remove some of the liquid from the slurry before the slurry is introduced into the high pressure device so that the slurry has a fourth liquid to material ratio at least about 0.25 less than the first ratio.
In one preferred embodiment of this invention, the above method is performed such that h) is practiced prior to g) so that a slurry having a third liquid-to-material ratio is introduced to the treatment vessel. This embodiment also preferably additionally includes i) removing excess liquid from the slurry during or shortly after the process of g), that is, while introducing the slurry to the treatment vessel, or shortly thereafter, and also j) combining the liquids removed at g) and i) and using at least some of the combined liquids as the pressurizing medium in d). Furthermore, j) preferably is practiced by monitoring the temperature of the combined liquids and regulating the flow of the liquids in h) and i) so that the temperature of the combined liquid is maintained below a specified value. The specified temperature value typically ranges from about 90 to 120xc2x0 C. depending upon the prevailing pressure in the feed system.
The first liquid-to-material volume ratio is used in slurrying the material in a) is typically greater than 2.75:1, that is, about 2.75 to 3.25 to 1.0. This ratio is typically required in order for the slurry pump, for example, a Hidrostal(copyright) screw-type-impeller slurry pump manufactured by Wemco, to operate properly. For other types of slurry pumps this L/W ratio may even be lower, for example, 2.50:1 or less. The second liquid-to-material ratio is typically greater than 2.50:1, for example about 5.0:1 or greater, preferably about 7.0:1 or greater, or even 9.0:1 or greater. The third liquid-to-material ratio is typically at least about 0.25 less than the second liquid-to-material ratio, most preferably at least about 0.50 less than the second liquid-to-material ratio.
The first pressure to which the slurry is pressurized typically is in the range of 1 to 7 bar gage; the second pressure is typically in the range of 5 to 15 bar gage.
According to another aspect of the invention there is provided a cellulosic fibrous material treating system comprising: A material slurry vessel. A high pressure transfer device including a low pressure inlet, low pressure outlet, high pressure inlet and high pressure outlet. The slurrying vessel operatively connected to said low pressure inlet and outlet. A treatment vessel connected to the high pressure outlet. Means for removing some liquid from slurry moving between the high pressure outlet and treatment vessel and circulating the removed liquid to the high pressure inlet, said means comprising the drainer described above. And, the system devoid of a connection from the treatment vessel to the high pressure inlet. The system may also include means for removing some liquid from the slurry between said slurrying vessel and low pressure inlet, and returning removed liquid to the slurrying vessel, said means comprising the drainer described above.
The present invention also includes a system for feeding comminuted cellulosic fibrous material to a treatment vessel having an inlet, comprising or consisting of: a first vessel containing a slurry of comminuted cellulosic fibrous material having a first liquid-to-material volume ratio; a high-pressure transfer device having an low-pressure inlet, a low-pressure outlet, a high-pressure inlet and a high-press outlet; means for pressuring and transferring the slurry from the first vessel to the low-pressure inlet of the high-pressure transfer device; means for diluting the slurry and transferring the slurry from the high-pressure outlet to the treatment vessel at a second liquid-to-material ratio, greater than the first ratio; and means for removing at least some of the liquid from the slurry located between the high-pressure outlet of the high-pressure transfer device and the treatment vessel inlet to provide a slurry having a third liquid-to-material ratio less than the second ratio to the inlet of the treatment vessel, said means comprising the drainer described above.
The first vessel is preferably a Chip Chute or Chip Tube provided by Ahlstrom Machinery. The high-pressure transfer device is preferably a High-pressure Feeder as sold by Ahlstrom Machinery. The means for pressurizing and transferring said slurry to the high-pressure transfer device may be a chip pump for pumping the slurry into the high-pressure transfer device or a pump (for example, a pump known as a Chip Chute Circulation Pump) for drawing the slurry into the high-pressure transfer device, or any other suitable conventional pressurizing device. The means for diluting the slurry and transferring the slurry from the high-pressure outlet of the high-pressure transfer device is preferably a high-pressure pump that provides pressurized liquid to the high-pressure inlet of the high-pressure transfer device. The preferred liquid-to-material ratios and pressures are as described above.
The above methods and apparatuses in which liquid is removed prior to introducing a slurry to the high pressure transfer device or liquid is removed after the slurry is discharged from the high-pressure transfer device can be used alone or in tandem. In either case, the flow of liquid from the two liquid removal devices is preferably controlled, for example, by appropriate valves, and in one embodiment the flows can be combined. The temperature of the individual liquids or of the combined liquid is preferably monitored and limited to a temperature that will prevent flashing of the liquid in the feed system. This is preferably effected by controlling the amount of liquid removed from the respective liquor separators, for example, by appropriate valves. The temperature of the liquids may also be controlled by passing one or more of the liquids through a cooling heat exchanger. This cooling heat exchanger may be used to heat other fluids, such as dilution liquids or cooking liquor, including kraft white liquor.
The present invention also includes a system for feeding a slurry of comminuted cellulosic fibrous material to a treatment vessel, comprising or consisting of: a first vessel containing a slurry of material and liquid having a top and a bottom, with an inlet adjacent the top and an outlet adjacent the bottom; a high-pressure transfer device having a low pressure inlet, a low pressure outlet, a high-pressure inlet, and a high-pressure outlet, the high-pressure outlet operatively connected to the treatment vessel; a pump, operatively connected to the outlet of the first vessel, for pressuring and transferring the slurry to the low-pressure inlet of the high-pressure transfer device; and means for removing liquid from the slurry located between the pump and the treatment vessel. The means for removing liquid from the slurry is distinct from the high-pressure transfer device and comprises a drainer as described above. The treatment vessel is preferably one or more continuous digesters, or one or more batch digesters, for producing cellulose pulp, and comprises a drainer as described above. The drainer may be located immediately upstream or downstream of the high-pressure transfer device, or two such drainers may be used: one upstream of the transfer device and one downstream.
In a preferred embodiment, the means for removing liquid from the slurry comprises a first means located near to or adjacent the inlet of the treatment vessel while the treatment vessel also includes a second means for removing liquid from the slurry. In this embodiment, the liquid removed from the first means and second means is combined and returned to the high-pressure transfer device. The first means is preferably a drainer as described above, and the second means is preferably a Top Separator, Inverted Top separator, or xe2x80x9cstilling wellxe2x80x9d arrangement located in the inlet of the treatment vessel, but other conventional devices may alternatively or additionally be utilized. The first and second means for removing liquid also preferably include a means for regulating the flow of liquid removed, for example, using conventional control valves. Also, the invention preferably includes means for measuring the temperature of the combined liquids, and means for regulating the flow of liquid from the first and the second means for removing liquid to maintain a specified maximum temperature of the combined liquids.
The system of the present invention preferably also includes a pretreatment vessel, for example, a steaming vessel, having an inlet and an outlet which communicates with the inlet of the first vessel. The pretreatment vessel is preferably a DIAMONDBACK(copyright) steaming vessel as sold by Ahlstrom Machinery and described in U.S. Pat. Nos. 5,500,083; 5,617,975; 5,628,873; and 4,958,741, or a CHISELBACK(trademark) vessel as described in co-pending application Ser. No. 09/055,408 filed on Apr. 6, 1998, now U.S. Pat. No. 6,189,288, though other more conventional screw-conveyor-type steaming vessels, or other conventional constructions, may be used. The system also preferably includes a metering device positioned between the pretreatment vessel and the first vessel. The metering device may be a star-type metering device, such as a Chip Meter as sold by Ahlstrom Machinery, or a screw-type metering device. In a preferred embodiment of the invention, the first vessel is a Chip Tube or Chip Chute as also sold by Ahlstrom Machinery.
These and other embodiments of this invention will become more apparent upon review of the following drawings and the attached claims.